U.S. Geological Survey
Miscellaneous Field Studies Map MF-2196
1992

By James J. Lienkaemper

Introduction

The purpose of this map (pl. 1) is to show the location
of and evidence for recent movement on active fault
traces within the Hayward Fault Zone. The mapped traces
represent the integration of three different types of data:
(1) geomorphic expression, (2) creep (aseismic fault
slip), and (3) trench exposures. The location of the
mapped area is shown on figure 1 on plate 1.

A major scientific goal of this mapping project was to
learn how the distribution of fault creep and creep rate
varies spatially, both along and transverse to the fault.
The results related to creep rate are available in
Lienkaemper and others (1991), and are not repeated
here. Detailed mapping of the active fault zone contributes
to a better understanding of the earthquake source
process by constraining estimates of: (1) the probable
recurrence times of major earthquakes, (2) the size of
expected surface displacements, and (3) the expected
length of ruptures accompanying these earthquakes.
Now that the 1989 Loma Prieta earthquake has occurred
on the San Andreas Fault, the Working Group on
California Earthquake Probabilities (1990) considers the
Hayward Fault the most probable source of a major earthquake
(magnitude 7 or larger) in the San Francisco Bay
region in the next few decades.

This map also is of general use to engineers and landuse
planners. The traces shown on the map are those that
can be expected to have the most intense ground rupturing
from fault slip in future major earthquakes on the
Hayward Fault. However, the small scale of the map
(1:24,000) does not provide sufficient details of the
local complexities of the fault zone for site development
purposes. The term "recently active fault trace" is
defined here as a fault trace that has evidence of movement
in the Holocene or approximately the last 10,000
years. This definition also satisfies the legal definition
of active fault used in the implementation of the Alquist-
Priolo Act of 1972 (Hart, 1990).

This map provides a starting point for planners by
showing locations of fault creep and trench exposures of
active traces. However, minor fault traces may not be
recognized because many sections of the fault were
already urbanized by 1939, the date of the earliest aerial
photography for the entire fault. Thus, geomorphic
features indicative of active faulting have been degraded
or destroyed by human activity, especially secondary
traces that have minor cumulative slip. For this reason,
subsurface investigations will continue to be the main
method of recognizing and precisely locating active fault
strands in sites that lack reliable creep evidence.
Mapping of creep evidence and monitoring of fault creep
can be the most definitive methods to precisely locate
active traces. Because important subsurface and creep
monitoring investigations are now in progress or
planned, this map must be considered an interim report
of data available on January 1, 1992.

Note: Note: MF2196 has been superceded by USGS Data Series 177. The map was originally printed with two colors. This site allows download of grayscale scanned images of the original paper map. Some variation from the original 1:24,000 scale may result from the scanning of a non-scale-stable materials.